Earth boring or contacting devices, such as rock bits used in petroleum and rock drilling applications, include wear or erosion surfaces exposed to erosive wear due to contact with geological formations. Two types of rock bits are commonly used: tungsten carbide inserts (TCI) rock bits and milled tooth rock bits.
A TCI rock bit is utilized to drill a hard formation because of the enhanced ability of tungsten carbide inserts to penetrate hard formations. However, the tungsten carbide inserts are mounted in a relatively soft metal, e.g., steel, that forms the body of the cutter cone. This relatively soft metal cutter body which holds the inserts in place may be abraded or eroded away when subjected to a high abrasive drilling environment. This abrasion or erosion occurs primarily due to the presence of cuttings from the formation, the direct blasting effect of the drilling fluid utilized in the drilling process, and the rolling and sliding contact of the cone body or cone shell with the formation. When the material supporting the inserts is eroded or abraded away to a substantial extent, the drilling forces being exerted on the inserts may either break the inserts or force them out of the cutter cone when they engage the formation. As a result, the bit may no longer be effective in cutting the formation. Moreover, the loose inserts that break off from the cutter cone may damage other inserts and the cutter cone, and eventually may lead to failure of the cutter cone.
When drilling relatively soft but abrasive formations, individual cutting inserts may penetrate entirely into the abrasive formation, causing the formation to come into contact with the cutter cone or cone shell. When this contact occurs, the relatively soft cone shell material will erode away, namely at the edges of the surface lands, until the previously embedded portion of the insert becomes exposed and the retention ability in the cone shell is reduced, which may result in the loss of the insert and reduction of the life of the bit. To protect the cutter cone from erosion, hardfacing material, such as tungsten carbide, has been applied to the cone surfaces by a variety of methods.
Milled tooth rock bits are another important type of rock bits used in petroleum and mining drilling applications. A milled tooth bit has a roller cone with teeth protruding from the surface of the cone for engaging the rock. The teeth are made of hardened steel and generally are triangular in a cross-section (taken in a plane perpendicular to the axis of the cone). The principal faces of such a milled tooth that engage the rock usually are dressed with a layer of hardfacing material to increase wear-resistance.
With respect to hardfacing cone surfaces of rock bits for erosion protection, different approaches have been developed with varying degree of success. For example, small, flat-top compacts made of hard material may be placed in the vulnerable cutter shell areas by a silicate bonding agent to prevent erosion. Thermal spraying, plasma arc, and welding arc also may be used to coat the exposed surfaces, including the inserts, of a cutter cone with a wear resistant material.
Although hardfacing coatings in accordance with existing methods protect cones from erosion to some extent, they are relatively unsatisfactory in their erosion protection performance. It is recognized that a good hardfacing coating preferably has a high carbide content and possesses strong bonding to the cone surface.
In a high energy deposition process, such as thermal spraying, plasma arc, and welding arc, dissolution of the carbides in the hardfacing material may occur extensively due to the long duration of high temperatures the carbide is subjected to. In addition, the high temperature in a plasma arc or welding arc process may cause the substrate metal to melt and diffuse into the hardfacing material. On the other hand, in thermal spraying and a low energy process (e.g., use of silicate bonding agents), the bonding between the hardfacing and the cone surface may be relatively weak.
Therefore, there exists a need for a method capable of depositing a hardfacing coating with a relatively higher carbide content and low substrate dilution while, at the same time, achieving strong metallurgical bonding to the substrate metal.